The present invention broadly relates to a new and improved construction of a headbox apparatus for a papermaking machine.
In its more particular aspects, the present invention specifically relates to a new and improved construction of a headbox apparatus for a papermaking machine which defines a predetermined machine direction and a predetermined web width and which produces a paper web from an infed fiber stock suspension flowing through such headbox apparatus. The headbox apparatus contains a distribution box or distributor for distributing an infed fiber stock suspension or pulp across the web width of the papermaking machine. A diffusor or guide system follows the distribution box or distributor and possesses a plurality of diffusor bores or channels for the infed fiber stock suspension and a subsequent nozzle chamber contains a delivery gap for distributing the infed fiber stock suspension at a predetermined weight per unit area across the web width defined by the papermaking machine. The weight per unit area or mass distribution across the web width of the papermaking machine is selectively variable by locally changing the opening width of the delivery slice or gap and/or by locally varying the stock density of the infed fiber stock suspension. The invention also relates to a method of operating such headbox apparatus.
Headbox apparatuses of such type are known, for example, from U.S. Pat. No. 4,087,321, granted May 2, 1978. They serve for infeeding a prepared fiber stock suspension into a papermaking machine and for distributing the infed fiber stock suspension in a predetermined manner across the entire web width of the papermaking machine. This distribution of the infed fiber stock suspension should be carried out such that the paper web produced by the papermaking machine has preselected parameters or characteristics across the web width. In most cases there is thus intended a uniform weight per unit area or mass per unit area after drying, a uniform dampness, as well as a uniform fiber orientation across the whole web width.
From U.S. Pat. Nos. 3,556,935, granted Jan. 19, 1971, and 4,089,739, granted May 16, 1978, it is known in this respect to construct the opening width of the delivery slice or gap of the nozzle chamber in an adjustable manner by using a number of adjusting devices distributed across the web width. There can be controlled thereby the quantity of the infed fiber stock suspension passed across the web width but not the fiber orientation.
Inadequacies based on the construction of the papermaking machine, as well as physical phenomena which occur during the papermaking process, are the reason that in most cases not all parameter values or characteristics across the web width occur simultaneously as desired or in a uniform manner. For example, deficiencies in the sieve part or shrinking of the paper web during drying, especially at the edge of the web, are compensated for by variation of the local stock mass flow just as inadequacies in the geometry of the distribution box or distributor, the diffusion or guide system or the nozzle chamber. For varying or adjusting the local stock mass flow, there is locally varied in many cases the opening width of the slice or outlet gap from the nozzle chamber.
However, the pressure in the nozzle chamber is thereby also locally varied and the pressure variation in the infed fiber stock suspension is non-uniform across the web width of the papermaking machine. These local pressure differences in the nozzle chamber result in transverse flows in the nozzle chamber and such transverse flows become effective up to the delivery slice or gap, particularly in a manner such that the direction of the flow in the nozzle chamber, as viewed across the web width, is not exactly parallel and does not conform with the direction of the papermaking machine. Even small deviations from the direction of the papermaking machine lead to undesired non-uniformities in respect of the fiber orientation in the produced paper web across the web width due to the transverse components of the velocity vector present during the outflow of the jet of the fiber stock suspension.
It is already known from Austrian Patent No. 363,776, granted Aug. 25, 1981, to balance pressure differences in the flow of infed fiber stock suspension in the distribution box or distributor across the web width by additionally infeeding or withdrawing fiber stock suspension into or from the infed fiber stock suspension at suitable locations, for example, in the distribution box or distributor so that the pressure variation in the distribution box or distributor is constant across the entire web width. However, solely using these measures, not all pressure differences in the nozzle chamber can be balanced.
It is known from U.S. Pat. No. 3,573,160, granted Mar. 30, 1971, to measure the velocity profile of the infed fiber stock suspension across the web width and to appropriately adjust the cross-section of the distribution box or distributor, e.g. by means of a displaceable wall, in order to achieve a uniform fiber stock suspension velocity in the delivery slice or gap. Pressure differences and transverse flows also can only be incompletely prevented by such adjustment.
As described in British Patent No. 1,216,114, it has already also been attempted to achieve a uniform flow of the infed fiber stock suspension by providing a number of individually adjustable overflow pipes at the distribution box or distributor. The aforementioned disadvantages also cannot be avoided thereby.
For example, additional pressure drops or losses occur in the nozzle chamber at its margins due to friction losses at the side walls. Such additional pressure drops or losses cause the pressure in the nozzle chamber and the throughput stock mass flow to decrease toward the aforementioned margins. As a result, transverse flows are also generated thereby in the nozzle chamber and result in flow lines which are no longer aligned exactly parallel to the predetermined direction of the papermaking machine.
It is proposed in German Patent Publication No. 2,151,906, published Apr. 26, 1973, that, for achieving uniform velocity and stock mass flow of the fiber stock suspension also in the marginal regions, there be provided at the distribution box or distributor a gap which is individually adjustable across the web width. Pressure differences and transverse flows in the delivery slice or gap, however, are also present in this construction.
Also in the case of a number of mutually offset rows of diffusor bores or channels in the diffusion or guide system as disclosed, e.g. in U.S. Pat. No. 4,137,124, granted Jan. 30, 1977, there are present in the marginal regions, non-uniformities such as smaller stock mass flows per unit width and such non-uniformities also lead to pressure differences and to transverse flows in the nozzle chamber and at the slice or gap. By infeeding additional fiber stock suspension or water at the sides through the side walls of the nozzle chamber, this could possibly be compensated for but is contingent upon a considerable complication in terms of machinery and control techniques.